In an apparatus such as a magnetic bearing type turbo-molecular pump which supports a rotor in a contactless manner by a magnetic bearing device, in order to maintain the rotor in a levitated state at a target position, the magnetic attraction force (electromagnet current) of an electromagnet is feedback-controlled in real time on the basis of the deviation (displacement) of the levitated position of the rotor from the target position.
In detection of the displacement, a method of performing the detection using a dedicated displacement sensor is common. However, in recent years, in order to achieve compactness and cost reduction, and improve the reliability, a sensorless type apparatus (also called a self-sensing type apparatus) has been coming into practical use. In the sensorless type apparatus, the dedicated sensor is omitted and an electromagnet which generates a levitation control force has not only a conventional actuator function, but also a sensing function (inductance system).
In the inductance system, a high-frequency carrier (sensor carrier) is applied to a dedicated sensor or an electromagnet coil to amplitude-modulate the sensor carrier by an inductance change caused by a levitation gap and then to demodulate the modulated sensor carrier, thereby obtaining a levitation gap signal (displacement signal). In the demodulation processing, there have been proposed many methods in which a digital technique is applied to synchronously sample and take in modulated wave signals by an AD converter, that is, many direct methods in which smoothing processing which may cause delay is not required.
A technique described in Patent Literature 1 (JP 2006-308074 A) relates to a configuration provided with a dedicated sensor. In this technique, the relationship between a sensor carrier frequency fc and a sampling frequency fs when sampling a modulated wave signal satisfies fs=2fc or fs=fc/n (n is a natural number). Since only a sensor carrier signal voltage is applied to the dedicated sensor, the S/N ratio of the signal is generally excellent. However, for example, when magnetic flux generated by a control current which excites an electromagnet affects a signal of a dedicated sensor coil such as a case where the electromagnet and the dedicated sensor are arranged in extremely close to each other in order to make an apparatus in which the magnetic bearing is mounted compact, a control current component (noise component) may be disadvantageously mixed to a signal component modulated on the basis of rotor displacement due to the influence of the magnetic flux.
Therefore, generally, most of the noise components are filtered by a band pass filter (a band pass filter having the sensor carrier frequency fc as the center frequency) provided immediately in front of an AD converter. However, in order to remove all of the noise components, it is necessary to make a Q value of the band pass filter further larger to achieve band-narrowing. However, when the band pass filter is made to have a narrow band, the demodulated displacement signal is largely delayed from the original signal, and the magnetic bearing control itself is deteriorated. Therefore application of the narrow band has a limit. Therefore, a noise component is left in the input signal of the AD converter, and the noise affects also the demodulated signal. Accordingly, a vibration component that is not actually displaced (vibrated) is mixed into the demodulated rotor displacement signal, and the displacement information thereof is fed back as it is to perform levitation control. As a result, the rotor is forcibly vibrated by the noise component, and the reaction force thereof is transmitted to the stator side, which may cause vibration of the apparatus.
A technique described in Patent Literature 2 (JP 2000-60169 A) relates to an apparatus provided with a dedicated sensor and a sensorless type apparatus. In the apparatus provided with a dedicated sensor, square wave signals inverted in sign at every sampling time under the condition of fs=2fc are generated by digital processing and output from a DA converter. Each of the square wave signals is modulated as a sensor carrier signal by a displacement signal (rotor displacement) by a sensor, and the modulated wave is taken in with the same frequency fs (=2fc) in synchronization with the peak timing. In demodulation processing, signal data taken in by an AD converter is processed by inverting the sign at every single sampling (inverting the sign at the minimum peak of the sensor carrier). Therefore, as with the invention described in Patent Literature 1, there is a problem of the generation of vibration.
Further, in the sensorless type apparatus, an electromagnet drive current signal on which a sensor carrier signal is superimposed is output from a DA converter, and the electromagnet is excited through a power amplifier. The superimposed sensor carrier signal is amplitude-modulated in the electromagnet coil. Therefore, the amplitude-modulated signal containing a displacement signal component is extracted, and demodulation processing in synchronization with the sensor carrier is performed in the same manner as in the apparatus provided with the dedicated sensor. However, in the sensorless type apparatus, a displacement signal is sensed by the electromagnet instead of the dedicated sensor. Therefore, not only the modulated signal of the superimposed sensor carrier signal, but also the control current signal is mixed in an equal or higher signal level. Therefore, the number of control current components (noise components) mixed into the amplitude-modulated signal is larger than that in the apparatus provided with the dedicated sensor.
A technique described in Patent Literature 3 (JP 2001-177919 A) relates to a sensorless type apparatus in which a sensor carrier component for sensing is superimposed on a drive current which excites an electromagnet. Basic signal processing is the same as that described in Patent Literature 2, and a different point is as follows. Specifically, sensor carriers (carrier waves) each of which is superimposed on each of a pair of electromagnets that face each other with a rotor interposed therebetween are applied in the opposite phase relationship. Accordingly, an amplitude-modulated signal containing a displacement signal component is efficiently separated and extracted from a control current component. However, the characteristics and the peripheral circumstances of one of the paired electromagnets never become completely the same as those of the other one of the paired electromagnets. Therefore, although there is a difference in degree, there is a problem of noise mixed into a displacement-modulated signal due to the same reason as in the sensorless type apparatus of Patent Literature 2.